In the 17th and 18th centuries, the Northern colonies were part of the British Empire, and slavery was legal in all British colonies. However, the northern colonies had different economic and social conditions than the southern colonies, which led to different patterns of slavery.
The Southern colonies were largely dependent on agriculture, and plantation owners relied on slave labor to cultivate crops such as tobacco and cotton. In contrast, the Northern colonies had more diverse economies, with trade, manufacturing, and shipping playing important roles.
As a result, slavery was less essential in the Northern colonies, and the number of slaves was much smaller than in the South.
Additionally, the religious and cultural beliefs of many northern colonists played a role in shaping the institution of slavery in the North. For example, the Quakers, a religious group that was influential in Pennsylvania and other Northern colonies, were opposed to slavery on religious grounds.
As a result, Pennsylvania became the first colony to pass a law against slavery in 1780, although the law was not fully effective until the 1790s. New York and Massachusetts also passed laws to abolish slavery in the late 18th century.